The automatic detection, classification and measurement of field work is of great help to farmers, not only for the automatic creation of compulsory documentation of their work for economical purposes and environmental protection, but in particular also for monitoring the work previously performed on the field and for planning future field works to be performed, whether ploughing, tilling, harrowing, preparation of seedbeds, cultivation, fertilisation, crop protection, cutting, turning, swathing, harvesting, etc. For this purpose, it is known to equip agricultural vehicles such as tractors, combine harvesters, etc. with satellite navigation receivers which record the path (trajectory) of the vehicle over the field so as to create the desired documentation and measurements on this basis. For distinction between actual work in the field and mere journeys to and from the field, digital maps are usually used, on which the fields are registered so as to assign the position specifications (position fixes) of the satellite navigation receiver to a field registered in the map.